Particulate aluminum oxide or alumina has long been employed as abrasive particles or grain in various abrasive products. The first source for alumina abrasive particles was the abundant supply which was found in nature. The naturally occurring alumina was later improved by fusion techniques, heat treatments and the addition of various additives. While such techniques have resulted in a dramatic improvement in the performance of abrasive articles which contain such abrasive particles, there still exists a great need for further improved alumina abrasive material to make it better able to withstand the reactivity and the abrasive wear caused by contact with a metal workpiece.
While at first blush one with a modicum of technical know-how, but not skilled in either the abrasive art or the ceramic cutting tool art, might look to the ceramic cutting tool art for guidance as to how abrasive particles may be improved to give them less reactivity and improved durability. For example, U.S. Pat. No. 4,366,254 describes a cutting tool of alumina, zirconia, and optionally a refractory metal compound such as a metal carbide, nitride, or carbo-nitride of the Group IVB and VB metals and the carbides of the Group VIB metals of the Periodic Table to provide a cutting tool which is tough without being as wear resistant as pure alumina. U.S. Pat. No. 4,543,343 describes a cutting tool comprising a high thermal conductivity ceramic material made of alumina, titanium boride, titanium carbide, and zirconia. U.S. Pat. No. 4,776,863 describes a cutting tool of hard metal which is coated with hard layers of titanium carbide, titanium carbonitride and/or titanium nitride, with an outermost thin layer of zirconium nitride. Successive layers of titanium carbonitride are coated via chemical vapor deposition (CVD) methods as is the zirconium nitride layer which according to the patent can also be deposited via physical vapor deposition (PVD) methods. PVD methods include evaporation by sputtering or by arc evaporation.
A multilayer coated cemented carbide cutting tool is disclosed in U.S. Pat. No. 4,746,563. This patent describes the use of CVD methods to form successive layers of alumina and metal nitrides or carbides on a cemented carbide substrate. The alumina layer with a total thickness ranging from 5 to 20 micrometers is divided into a plurality of layers each having a thickness ranging from 0.01 to 2 micrometers (10 to 2000 nanometers) by use of interlayers each of which has a thickness of 0.01 to 2 micrometers and consists of at least one member selected from the group consisting of TiC, TiN, TiCN, TiCNO, TiCO, TiNO, Ti oxides, Ti(B,N), Ti(B,N,C), SiC, AlN, and AlON.
Sputtering with a coating metal such as molybdenum has been used to coat diamond with an adherent metal layer to promote the bonding between diamond particles when formed into a compact, as described in U.S. Pat. No. 3,351,543. The cited advantage was to be able to bond together small synthetically made diamonds of variable sizes into a usable tool. Sputtering was used to clean the surface of the diamonds and then to coat with molybdenum. Coating thickness are reported as ranging from about 58 nanometers to 200 nanometers.
While certain of the aforementioned references indicate that improved physical properties may be obtained in ceramic cutting tools by the coating of such ceramic tools with refractory metal compounds, such disclosure does not direct the person skilled in the abrasive art to make similar modifications of alumina abrasive particles or grains.
Cutting tools cannot be equated with abrasive grain for several reasons. Grinding was once considered to be a purely mechanical action of metal removal much as a cutting tool. This was supported by the fact that the two commonly used abrasives, alumina and silicon carbide (SiC), behaved differently on different materials. Alumina, believed to be the harder mineral, was effective on high tensile steels while SiC was more effective on low tensile materials. With further study of these materials, it was determined that SiC was actually the harder of the two minerals. With the development of B.sub.4 C, which was a much harder than either alumina or SiC, it was found that this new mineral was inferior to both alumina and SiC in grinding steels. The theory that grinding was totally a mechanical process fell apart.
The reason why grinding is not totally a mechanical process may be found in comparing the distribution of energy used in metal removal by cutting tools and abrasive grinding. In cutting tools it has been estimated that up to 90% of the total energy used for cutting is removed with the chips and that only about 5% of this total energy goes into the metal surface as heat. As a result, the cutting tool temperature remains relatively low, about 700.degree. to 800.degree. C. for normal cutting speeds. For abrasive grinding, the total energy input into the operation is up to ten times greater than with cutting tools. And of the total energy input, about 80% goes into the workpiece at the grinding interface as heat compared with 5% for cutting tools. The energy going into the workpiece as heat is thus 160 times greater in the case of abrasive grinding than cutting tools. The reason for this difference may be found in the different mechanisms of chip formation and the values of rake angle. In cutting tools the rake angle is near zero allowing almost complete freedom for upward flow and removal of the chip. In abrasive grinding, rake angles have large negative values and there is considerable resistance to upward flow. As a result, considerable energy is spent in deforming the surface in grinding while little energy is removed with the chips. The temperature of the grinding interface thus reaches very high values, and may even reach the melting point of the metal as evidenced by solidified chips often found in the grinding swarf. Another indication of the high temperatures present is the spark shower observed in grinding which consists of chips heated to red or white heat.
As a result of the high temperatures encountered at the grinding interface, the interaction between the metal, the abrasive grain and the atmosphere must be considered. For example, the best explanation of why aluminum oxide is a better abrasive on most steels than silicon carbide is that a chemical reaction occurs at the high temperatures encountered in abrasive grinding between the silicon carbide and the steel which in effect "melts" the abrasive and causes excessive wear. Tungsten carbide, boron carbide and titanium carbide are also examples of very hard materials which have been found to be excellent cutting tool materials, yet have little utility as an abrasive grain due to their reactivities with various metals.
Thus, attempts at improving the grinding performance of abrasive grain for use in abrasive products such as coated abrasives generally have been directed away from substituting the bulk materials used in cutting tools for abrasive grain, and instead have focused on either changing the composition of the grain or on applying thin coats of refractory material to the grain.
U.S. Pat. No. 4,788,167 is an example of the former. In this patent, there is disclosed an abrasive grain comprising aluminum nitride, aluminum oxynitride, and Periodic Group IVB metal nitride. The compositions described offer improvement over known compositions in grinding cold-rolled steel.
An example of the latter for various types of abrasive constructions is disclosed in laid open Patent Publication JP1-113485, published May 2, 1989, which describes alumina, zirconia, or silicon carbide abrasive grain coated with diamond or cubic boron nitride via chemical vapor deposition processes for use in grinding wheels, cutting blades, and finishing work. This publication is directed to conversion of the abrasive grains to "superhard" grains by coating them with diamond or boron nitride in thicknesses ranging from 0.5 to 10 micrometers (500 to 10000 nanometers).
U.S. Pat. No. 4,505,720, assigned to the assignee of the present application, describes an improved granular abrasive mineral made by coating hard refractory material onto silicon carbide abrasive grain. The coatings are applied by chemical vapor deposition onto silicon carbide grit in a fluidized bed. The resultant coated grain offers a significant increase in abrasive performance when used to grind steel.
What has been lacking in the art, however, is an improvement in the grinding characteristics of alumina-based abrasive grain for use in coated abrasives to grind metal.
One reason metal removal by alumina-based coated abrasives is generally at a low rate is probably due to "metal capping" of the abrasive particles. "Metal capping" is the term used to describe the coating of abrasive particles by metal from a workpiece during abrading. Metal capping dramatically reduces the effectiveness of a coated abrasive product. It is a particularly bothersome problem when fine-grade abrasive particles (&lt;100 mesh or 150 micrometers in average particle size) are used. It is believed that elimination or reduction of the metal caps would improve the grinding rate and increase the useful life of the abrasive mineral.